This invention relates to a silicon based negative electrode for lithium-ion batteries where the association of micro-sized silicon, an electrode fabrication process and additives in electrolyte allows better electrochemical performances.
Lithium-ion batteries are the most widely used secondary systems for portable electronic devices. Compared to aqueous rechargeable cells, such as nickel-cadmium and nickel metal hydride, Li-ion cells have a higher energy density, higher operating voltages, lower self discharge and low maintenance requirements. These properties have made Li-ion cells the highest performing available secondary batteries.
The worldwide energy demand increase has driven the lithium-ion battery community to search for new generation electrode materials with high energy density. One of the approaches is to replace the conventional carbon graphite negative electrode material by another better performing active material, being a metal, metalloid or metallic alloy based on silicon (Si), tin (Sn) or aluminum (Al). These materials can provide much higher specific and volumetric capacity compared to graphite. On top of the specific composition of the negative electrode material, the surface properties of the particles—being constantly in contact with a reactive electrolyte—are playing a key role in the electrochemical behaviour of the resulting Li-ion battery.
As mentioned above, Si-based negative electrode materials could significantly enhance the energy density of the commercial lithium ion batteries. Silicon has the largest theoretical gravimetric capacity (3579 mAh/g) corresponding to the following reaction: 15Li+4Si→Li15Si4, and a large volumetric capacity (2200 mAh/cm3). However, the huge volume expansion upon lithium intercalation had never allowed reaching acceptable life characteristics for a use in rechargeable cells.
The synthesis of materials at the submicron or nanoscale is generally the solution to overcome the main drawbacks of these materials, and makes them suitable candidates for the replacement of graphite anodes. An interesting method to prepare submicron powders is plasma technology, as is disclosed in WO2008/064741 A1. Unfortunately, even for nanometric materials the high BET surface and the creation of a new silicon surface after each volume expansion causes a continuous decomposition of the battery's electrolyte, that results in a low coulombic efficiency of this material.
The use of binders favouring a more resilient bonding between the Si and the conductive carbon black particles than the standard polyvinylidene fluoride (PVDF) is also one of the favourite strategy to improve the electrode cohesion during the volumetric variation.
And finally some strategies use electrolytes containing a film-forming agent to improve the behaviour of the solid electrolyte interface (SEI) and thus to limit the continuous electrolyte degradation. Fluoro ethylene carbonate (FEC) and vinylidene carbonate (VC) are known examples of such SEI film-forming agents, see for example N-S. Choi et al. in J. Power Sources, 161 (2006) 1254. Flexible polycarbonate would be the major surface film component in FEC and VC-containing solutions. They would have a better ability to accommodate the volume variations of the Si phase, thus limiting the contact between the electrode and the liquid electrolyte. This would reduce the amount of SEI products precipitating and accumulating inside the electrode at each cycle.
With respect to the selection of the binder and the electrode processing, several publications and patents highlighted improvements of the capacity retention of silicon-based electrodes prepared with a carboxymethylcellulose (CMC) binder.
The suitability of the CMC binder for silicon based-anodes has many different explanations, including the presence of original and indispensable polymer-particle interaction. The CMC-efficiency could be ascribed to its extended conformation in solution, which facilitates the formation of an efficient network. The slurry pH during the electrode preparation (typically around pH 3) was also demonstrated to be a key parameter to obtain astonishing cyclability improvements, as disclosed in D. Mazouzi et al. in J. Electrochem. Solid-State Lett. 12 (2009) A215. This results on the one hand from the physical cross-linking of the CMC chains in the solution at pH 3, and on the other hand from an ester-like Si—CH3COO—R covalent bond between the binder and the silicon surface, by the reaction of the CMC carboxyl groups with the OH groups at the surface of the thin SiO2 layer surrounding the Si particles.
It is an object of the present invention to further improve the cycling performance (more in particular the capacity retention) of Si based negative electrode materials, and in the meantime offer a cheaper material that exhibits the same and even better performances than the known Si nanomaterials.